In single mode optical fibers, a significant fraction of the total guided energy is not confined to the core, and the “tail” of the power distribution extends a considerable distance into the cladding material. Since the substrate (or sleeve) tube material outer cladding is typically less pure (and therefore much more lossy) than the cladding material, it is necessary to ensure that no significant fraction of the total power propagates in the substrate- or tube-derived material.
In one type of prior art single mode fiber, referred to as a “depressed cladding” fiber, the effective refractive index of the cladding material is chosen to be substantially less than the refractive index of the core. In most of these depressed cladding prior art designs, the core region is “up doped” and the cladding region is “down doped” so as to obtain the largest difference in refractive index with the smallest overall fiber diameter. The ratio of the cladding diameter D to the core diameter d, is used in determining various performance parameters of optical fiber made from the perform. For example, to obtain optical fiber having desired transmission characteristics, the D/d ratio should be within an acceptable, but relatively narrow, range of values. The single mode cut-off wavelength must also be taken into account in the determination of the appropriate D/d value. The cut-off wavelength is the wavelength below which the optical fiber behaves as a step-index multimode fiber and above which behaves as a single mode fiber. Also, the D/d ratio affects the mode field diameter (MFD) which is a measure of the width of the light intensity in a single mode fiber—also referred to as the “spot size”. In most cases, it is desired to maintain the ratio D/d less than 2.5 While this value is acceptable for most short wavelength arrangements, long wavelengths (e.g., 1550 nm) cannot be supported in such an arrangement.
In the case where a depressed clad/pure silica core fiber is used (i.e., cladding is doped to exhibit a refractive index less than silica, a non-negligible fraction of the total power will to leak to the outer cladding. The fiber would thus have relatively high loss, even if the outer cladding has a low absorption loss, comparable to that of the deposited cladding material. This type of loss is referred to as a “leaky mode” loss, since the radiation propagating in the outer cladding is unguided and will “leak” away. Leaky mode loss can be avoided by depositing a significantly thick cladding layer.
Therefore, for a pure silica core fiber (such as fabricated by MCVD), the depressed cladding which provides the index difference necessary for a waveguide must be large enough to contain the single mode, while not allowing the energy to leak from the fiber and drastically increase attenuation at the specified wavelength. Furthermore, the perform must be designed to have a cutoff wavelength that is relatively close to the operating wavelength to adequately contain the mode. Further, the depressed cladding material should have a thickness sufficient to contain the operating wavelength mode without suffering from huge bending loss.